Rapid and Continuous Monitoring Method of Radon (²²²Rn) for Seawater Using a Pulsed Ionization Chamber

In the marine environment, 222Rn has been widely utilized as a valuable tracer for measuring the exchange rates and fluxes of substances. However, owing to the extremely low activities of 222Rn in the ocean, applications have been limited by sample collections and measurements. Recent studies have proposed a continuous monitoring method by connecting a water–air exchanger to a commercially available radon-in-air monitor (RAD7, Durridge Co.) to reduce labor. However, this method still suffers from the low measurement efficiency and long measurement time for seawater. Therefore, in this study, we developed a new continuous monitoring system by using a new radon-in-air monitor (FRD2400, FT-lab Co.), based on a pulsed ionization chamber (PIC), and water–air exchangers. This system demonstrated a faster equilibrium time (<10 min) and approximately eight times higher measurement efficiency than the RAD7-based continuous monitoring system. With this system, together with a nitrate sensor, we successfully obtained high-resolution 222Rn and nitrate data in coastal waters, locating groundwater-borne sources. Thus, this newly developed system will facilitate the rapid and easy measurement of 222Rn in various marine environments, promoting the active utilization of 222Rn tracers for understanding coastal processes.